This research project includes an investigation of the mechanisms of physiological control of the early events in the classical complement pathway. Additionally, clinically useful methods will be developed for the detection and quantitation of complement activation in human serum. The mechanisms of control of C1 activation and C1 activity will be studied. The classical complement pathway will be reconstituted from the purified proteins, C1q, C1r, C1s, C1-Inactivator, C2, C3, C4, beta-1H, C3b inactivator, and C4 binding protein. The kinetics of activation and control of this purified system will be studied and compared to normal human serum. These studies will aid in the understanding of the interactions of the known complement components and control proteins. Additionally, if the data indicate that there are new serum principles, they will be isolated and characterized. The influence of the nature of the activator on the interaction of C1-Inactivator (C1-In) with C1 will be investigated. The highly specific proteolysis of C1r and C1s in C1rC1s(C1-In)2 will be studied. Furthermore a mechanism of C1-Inactivator induced recycling of C1q will be proposed and investigated. Assays to detect and quantitate C1 activation in human serum will be developed and critically evaluated. One assay is based on the antigenic modulation of the C1r subunit of C1 with activation of C1 in serum. A second test will be a radioimmunoassay that measures the concentration of C1rC1s(C1-In)2, which is a product of complement activation. These tests will be evaluated for their ability to assess C1 activation in pathological human sera, and to quantitate serum levels of immune complexes and other C1 activators (e.g., C-reactive protein, DNA, etc.).